Flattening Filter Free (FFF) IMRT Reduces Secondary Neutrons at 6 and 10 MV Beam Energy

Abstract

High-energy photons cause formation of secondary neutrons relative to their energy by interaction with beam-forming structures in the LINAC-head and in tissue. Their fraction in the therapeutic beam should be kept to a minimum due to their potentially dangerous effects towards generation of secondary tumors and deleterious effects on implanted electronic devices. We investigated therefore, if flattening-filter free (FFF) IMRT with 6 and 10 MV would result in decreased formation of secondary neutrons in comparison to conventional IMRT. We used a LINAC with 160-leaf MLC to irradiate IMRT plans with identical dose distributions and therapeutic efficacy for 6 and 10 MV with and without flattening filter. Plan a) Emulation of large volume pelvic RT for lymphatic metastasized prostate cancer, single dose 2 Gy. Plan b) Intensely modulated small volume plan to prostate/seminal vesicles with integrated prostate-boost, single doses 2.25/3 Gy. All plans were applied to a RW3-slate phantom. Secondary neutrons were measured with a reference-neutron dosimeter in 40 cm distance cranial to the isocenter. All 6 MV plans resulted in significantly reduced neutron readings in comparison to 10 MV. FFF-IMRT resulted even at 6 MV in further significant reduction neutron readings in comparison to conventional IMRT. With 10 MV, FFF-IMRT resulted in significant reduction in neutron dose in comparison to conventional IMRT in plan A). For the heavily modulated plan B), treating in FFF-mode did not result in a decreased rate of secondary neutrons. As expected, reduction of beam energy from 10 to 6 MV resulted in significantly decreased measurements of secondary neutrons. At 6 and 10 MV, FFF-IMRT results especially in less modulated IMRT-plans in significantly reduced secondary neutron readings in comparison to conventional IMRT. In intensely modulated IMRT-plans, extensive interaction of 10 MV photons with the MLC obscures the neutron-sparing effect of FFF-IMRT. FFF-IMRT together with the use of lowest-possible beam energy therefore ensures low patient neutron exposure in most clinical situations.